Police nationwide shot and killed 492 people in the first six months of this year, a number nearly identical to the count for the same period in each of the prior two years.

Fatal shootings by police in 2017 have so closely tracked last year’s numbers that on June 16, the tally was the same. Although the number of unarmed people killed by police dropped slightly, the overall pace for 2017 through Friday was on track to approach 1,000 killed for a third year in a row.

The Washington Post began tracking all fatal shootings by on-duty police in 2015 in the aftermath of the 2014 killing in Ferguson, Mo., of Michael Brown, who was unarmed and had an altercation with the officer who shot him. The ongoing Post project has documented twice as many shootings by police in 2015 and 2016 as ever recorded in a single year by the FBI’s tracking of such shootings, a pattern that is emerging again in 2017.

Since Brown’s killing in Ferguson, other fatal shootings by police, many captured on video, have fueled protests and calls for reform. Some police chiefs have taken steps in their departments to reduce the number of fatal encounters, yet the overall numbers remain unchanged.

Academics who study shootings give weight to The Post’s accounting.

“These numbers show us that officer-involved shootings are constant over time,” said Geoffrey Alpert, a criminologist at the University of South Carolina who has studied police use of force. “Some places go up, some go down, but it’s averaging out. This is our society in the 21st century.”

As in previous years, the data gathered by The Post showed that police most frequently killed white males who were armed with guns or other kinds of weapons. One in four people killed this year were mentally ill. And police have continued to shoot and kill a disproportionately large number of black males, who account for nearly a quarter of the deaths, yet are only 6 percent of the nation’s population.

This year, fatal shootings of unarmed people have declined, continuing a trend over the past two years. In the first six months of this year, 27 unarmed people were fatally shot, compared with 34 for the same period in 2016 and 50 in the first six months of 2015.

Black males continued to represent a disproportionately large share of unarmed people killed, although their share has dropped slightly: from 32 percent of all unarmed killings during the first six months of last year to 26 percent for the same period this year.

Mourners emerge from the funeral for 15-year-old Jordan Edwards at Mesquite Friendship Baptist Church in Mesquite, Tex., on May 6, 2017. (Louis DeLuca/AP)

One of those seven unarmed black males killed was Jordan Edwards, a 15-year-old high school freshman who was shot in April by a police officer in a Dallas suburb. An officer in Balch Springs opened fire with an AR-15 rifle on Edwards and his friends as they drove away from a party, according to news reports. The department initially said the teens tried to back over the officer but retracted the statement after officials reviewed video of the shooting. The officer, who is white, has been fired and charged with murder.

A woman holds a sign in honor of Charleena Lyles during a protest and rally in Seattle on June 20, 2017. (David Ryder/Getty Images)

Mental illness has remained a factor in fatal police shootings, as a quarter of those killed were struggling with some form of mental illness. Last month, Seattle police shot and killed Charleena Lyles, 30, a pregnant woman suffering from mental illness, after she called 911 to report an attempted burglary at her home. Police said Lyles pulled a knife on two officers, who both shot her. The Seattle Times reported that one of the officers, trained to use a Taser, was not carrying it, a violation of the department’s policy.

Chuck Wexler, executive director of the Police Executive Research Forum, which develops training programs and advises police chiefs around the country on policy, said some fatal shootings can be eliminated.

“We know we can make a difference in cases where the person is mentally ill and in cases where someone is not armed with a gun,” Wexler said.

The study by The Post has found that about 8 percent of the nation’s police departments have had at least one fatal shooting since 2015. Of those, most had only one.

“All deadly force scenarios are different, and you could have five in a week and then not have any for a year,” said Rodolfo Llanes, chief of the Miami Police Department.

Llanes noted that his department of 1,250 officers has gone as long as a year without firing a single bullet. “[But] there will be that situation where there is a confrontation and deadly force is used. Police work is inherently ugly,” he said.

The pace at which officers have been killed in the line of duty has held steady over the past two years.

According to the FBI, 21 police officers were killed from January to June 29, two fewer than in the same period last year. The 2016 year ended with 66 officers killed, not including accidental deaths. Since January 2015, according to the FBI, 128 police officers have been killed in the line of duty.

Since 2015, The Post has created a database cataloging every fatal shooting nationwide by a police officer in the line of duty.

In Tulsa, police shot and killed one person in an 18-month stretch, and then, in the second half of 2016, officers shot and killed six, including Terence Crutcher, an unarmed black man whose death led to criminal charges against an officer. Last month, that officer was acquitted of a charge of first-degree manslaughter.

This year, Tulsa officers have fatally shot four people.

Tulsa Police Chief Chuck Jordan said that his agency reviewed its use-of-force policy after the surge in shootings in 2016 but that there was not much the department could change.

“We’re a reflection of the society we live in,” Jordan said.

In Los Angeles from January 2015 to the end of last month, city police officers shot and killed 47 people, the most for any U.S. police department in the period.

Officials with the Los Angeles Police Department said they have been working to reduce the number of deadly encounters. In the past two years, the department extensively tracked use of force, added training and updated its use-of-force policy, which now requires officers to “de-escalate” confrontations before firing their guns.

This year in Los Angeles, fatal shootings are down to seven, which Matthew Johnson, president of the Los Angeles Police Commission, considers a small victory.

“All these things in concert are going to have an impact on the need to use deadly force, but that’s not to say we’re not going to have a bad quarter or a bad year,” said Johnson, who made reducing the number of fatal shootings a priority when he took over as president of the commission in 2015.

About 360 miles away, Phoenix police have emphasized reforms but are leading the nation in fatal shootings in 2017. Officers there have fatally shot eight people, more than any other department.

The reforms began in 2014, after a year in which the Phoenix Police Department said it experienced an unusually high number of fatal and nonfatal shootings by its officers. The agency began an unprecedented review of those violent confrontations, modeling some of its reforms on those used in Las Vegas, where police reduced shootings from 11 in 2015 to three in 2016. This year, Las Vegas has had five fatal shootings by police.

Phoenix police were retrained in tactics to de-escalate encounters and avoid resorting to deadly force.

On June 23, a Phoenix police officer fatally shot a motorist when the driver accelerated toward him as the officer approached the car on foot. Two days later, police killed a man who fired a rifle at a police helicopter and pointed the weapon toward officers.

In February, Phoenix Police Chief Jeri Williams defended her department in an op-ed for the Arizona Republic: “None of our officers wanted to be put into a position where they felt they had no other choice but to use deadly force.” She wrote that in each shooting, her officers had exhausted other options. “It was their last resort.”

There is no comprehensive government data source that tracks fatal shootings by police officers. The Post database relies on local news coverage, public records and social-media reports to identify fatal shootings by police.

Academics caution that fatal shootings are rare events in the universe of police-civilian interactions and say that more-complete data is needed about all police use of force.

“What we really need to know is how many times police shoot people, not just how many of those people die,” said David A. Klinger, a criminal justice professor at the University of Missouri in St. Louis who studies police use of force.

The FBI gathers information on fatal police shootings, but that program is based on voluntary reporting by police agencies and covers only cases inwhich police fatally shoot people who are committing felonies. The Post’s data has revealed a dramatic undercount by the FBI.

The Post’s project, and a similar counting effort by the Guardian newspaper in 2015, prompted now-fired FBI director James B. Comey to call his own agency’s system“embarrassing and ridiculous.” In October 2016, the Justice Department announced that it would move forward with plans to collect better data about officer-involved shootings.

The FBI said it would launch Saturday a pilot study of that data collection program that will gather a broad range of information on use of force from about 50 local and federal law enforcement agencies. The FBI said it intends to begin nationwide collection of the data in 2018.

“When a police officer takes a life, that’s a significant event,” said Darrel Stephens, the executive director of the Major Cities Chiefs Association. The group, which includes 80 police chiefs and sheriffs, helped advise the FBI on the pilot program. “We should know on a national basis how many times that happens and under what circumstances.”

Samuel Walker, an expert on police accountability and a criminal justice professor at the University of Nebraska in Omaha, said that because of the large number of U.S. police agencies, years of reforms may be required before the number of shootings declines.

“There is no single quick fix,” Walker said. “You have to have a really systemic approach.”

Ted Mellnik, Steven Rich, Jerrel Floyd and Catherine York contributed to this report. This article was produced in partnership with the Investigative Reporting Workshop at American University, where Thebault, York and Floyd are students.